1. Stomatal Conductance and Porometry
Theory and Measurement

2. Stomatal conductance Describes gas diffusion through plant stomata
Plants regulate stomatal aperture in response to environmental conditions
Described as either a conductance or resistance
Conductance is reciprocal of resistance
1/resistance

3. Stomatal conductance
Can be good indicator of plant water status/stress
Many plants regulate water loss through stomatal conductance

4. Fick's Law for gas diffusion
E Evaporation (mol m-2 s-1)
C Concentration (mol mol-1)
R Resistance (m2 s mol-1)
L leaf
a air

5. Cvt rvs Cvs rva Cva stomatal resistance of the leaf
Boundary layer resistance
of the leaf

6. Do stomata control leaf water loss?
Bange (1953)
Still air: boundary layer resistance controls
Moving air: stomatal resistance controls

7. Obtaining resistances (or conductances)
Boundary layer conductance depends on wind speed, leaf size and diffusing gas
Stomatal conductance is measured with a leaf porometer

8. Measuring stomatal conductance – 2 types of leaf porometer
Dynamic - rate of change of vapor pressure in chamber attached to leaf
Steady state - measure the vapor flux and gradient near a leaf

9. Dynamic porometer Seal small chamber to leaf surface
Use pump and desiccant to dry air in chamber
Measure the time required for the chamber humidity to rise some preset amount
Stomatal conductance is proportional to:
ΔCv = change in water vapor concentration
Δt = change in time

10. Delta T dynamic diffusion porometer

11. Steady state porometer
Clamp a chamber with a fixed diffusion path to the leaf surface
Measure the vapor pressure at two locations in the diffusion path
Compute stomatal conductance from the vapor pressure measurements and the known conductance of the diffusion path
No pumps

12. Steady state porometer
A chamber with a fixed diffusion path is clamped to the leaf surface
Steady-state technique; measures vapor pressure at two locations in a fixed diffusion path
Calculates flux and gradient from the vapor pressure measurements and the known conductance of the diffusion path.
Teflon
filter
Desiccant
Atmosphere

13. Decagon steady state porometer
Model SC-1

14. Environmental effects on stomatal conductance: Light
Stomata normally close in the dark
The leaf clip of the porometer darkens the leaf, so stomata tend to close
Leaves in shadow or shade normally have lower conductances than leaves in the sun
Overcast days may have lower conductance than sunny days

15. Environmental effects on stomatal conductance: Temperature
High and low temperature affects photosynthesis and therefore conductance
Temperature differences between sensor and leaf affect all diffusion porometer readings. All can be compensated if leaf and sensor temperatures are known

16. Environmental effects on stomatal conductance: Humidity
Stomatal conductance increases with humidity at the leaf surface
Porometers that dry the air can decrease conductance
Porometers that allow surface humidity to increase can increase conductance.

17. Environmental effects on stomatal conductance: CO2
Increasing carbon dioxide concentration at the leaf surface decreases stomatal conductance.
Photosynthesis cuvettes could alter conductance, but porometers likely would not
Operator CO2 could affect readings

18. What can I do with a porometer?
Water use and water balance
Use conductance with Fick’s law to determine crop transpiration rate
Develop crop cultivars for dry climates/salt affected soils
Determine plant water stress in annual and perennial species
Study effects of environmental conditions
Schedule irrigation
Optimize herbicide uptake
Study uptake of ozone and other pollutants

19. Case study #2 Washington State University wheat
Researchers using steady state porometer to create drought resistant wheat cultivars
Evaluating physiological response to drought stress (stomatal closing)
Selecting individuals with optimal response

20. Case study #3 Chitosan application
Evaluation of effects of Chitosan on plant water use efficiency
Chitosan induces stomatal closure
Leaf porometer used to evaluate effectiveness
26 – 43% less water used while maintaining biomass production

21. Case Study 4: Stress in wine grapes